What makes you think you overwatered them? If they're in well-draining soil then you probably don't have to worry. I've just been planting and watering in pre-started dahlias. Mother Nature added another inch+ of water and I'm not going to be worried until there are puddles standing in the holes.

Oooh, now I think I'll have to get Jitterbug! As to getting blooms already...I start mine in pots in the garage, then they move to the house when they get above the dirt, then to the greenhouse, and then outside the end of May/beg. of June. I have at least 6 or 7 that generous gardenwebbers sent me that I probably won't see blooms on this year. If we don't have them started early, we don't see blooms until the end of summer, and even though they are beautiful, it just isn't worth the garden space for only a week or two of bloom. Bahama Mama is actually much prettier than the pic I posted...for some reason some of the mauve came out brown with my Iphone.

They should if they survive. Dahlias grown from seed produce tubers their first year.

Well, clean 'em off and try it out. If that's just dirt, it's fine. If the black is rotting, it's not fine, but it's hard to tell from the photo (some of it does seem to be dirt--that furthest right one looks dirty but just fine!). And yes, temperatures under freezing at the tuber level will destroy them in our zones. BTDT, and unfortunately had to replace the tubers. Fortunately, you're not reporting temperatures in that regime, but FYI. They definitely can't be wintered at temps under freezing regardless of whether you can keep them dry or not around here ("here" being the Northeast right down to Philadelphia). Before storage, inspect and pinch a bit like you would a questionable potato. :-) Cut off anything that you think is rotted so it doesn't infect the rest of the mass.